Magnetic recording media generally comprise a magnetic layer coated on at least one side of a nonmagnetizable support. For particulate magnetic recording media, the magnetic layer comprises magnetic particles dispersed in a polymeric binder. The magnetic layer may also include other components such as lubricants; abrasives; thermal stabilizers; antioxidants; dispersants; wetting agents; antistatic agents; fungicides; bactericides; surfactants; coating aids; nonmagnetic particles; and the like.
The magnetic layer of a majority of conventional magnetic recording media is prepared by combining the magnetic particles, the polymeric binder, and other components (if any) with an organic solvent to form a dispersion. The dispersion is generally homogenized by lengthy mechanical milling to break up agglomerates of, and disperse, the magnetic particles. The resulting dispersion is then coated onto the nonmagnetizable support. The magnetic particles may then be magnetically oriented after which the coating may be dried, calendered, and/or cured.
The quality of the dispersion plays a vital role in determining the electromagnetic performance of the resulting magnetic recording medium. If the magnetic particles are insufficiently dispersed in the polymeric binder and other ingredients of the dispersion, electromagnetic and physical properties of the magnetic layer, such as the signal to noise ratio, squareness ratio, ability to reproduce short wavelengths, magnetic particle packing density, Young's modulus, surface smoothness, abrasion resistance, durability, and the like, tend to suffer dramatically.
Uniform dispersion of magnetic particles in the dispersion is very difficult to achieve, particularly when extremely fine, platelet-shaped magnetic particles, e.g., barium ferrite particles, are used. Organic dispersing agents have been used to facilitate the dispersing of the magnetic particles. However, due to the magnetic attractive forces between magnetic particles, there is a strong tendency for magnetic particles to reagglomerate in the dispersion when milling stops, even when organic dispersing agents are used. Additionally, the performance of organic dispersing agents at dispersing the extremely fine, platelet-shaped magnetic particles having a particle size of about 300 nm or less has been disappointing. To be effective at dispersing such particles, dispersing agents have had to be used in amounts that tend to weaken the adhesion of the magnetic layer to the nonmagnetic substrate, thus adversely affecting the abrasion resistance and durability of the medium. For these reasons, new technology for improving the dispersibility of magnetic particles is highly demanded.
Covering magnetic particles with an inorganic coating has been used to reduce the tendency of magnetic particles to agglomerate or aggregate. For example, U.S. Pat. No. 4,707,593 describes a visible image magnetic card in which magnetic particles and a fine fluidizing powder are combined in a display cell. A portion of the fluidizing powder covers the surfaces of the magnetic particles. The fluidizing powder reduces the adhesion and friction coefficient between the magnetic particles. The fine fluidizing powder is selected from materials which may include aluminum oxide.
U.S. Pat. No. 4,438,156 describes a method for coating magnetic particles with colloidal silica particles. To prepare the coated magnetic particles, the magnetic particles are mixed with an acid at a pH such that the magnetic particles have a significant, positive electrostatic charge. The pH of a slurry of the colloidal silica particles is adjusted so that the colloidal silica particles have a significant, negative electrostatic charge. When the slurry of colloidal silica particles is added to the acidic mixture of magnetic particles, the colloidal silica particles with their negative electrostatic charge are attracted to and coat the magnetic particles with their positive electrostatic charge.
U.S. Pat. No. 5,039,559 describes coating magnetic particles with inorganic oxides, which may include aluminum oxide. The particles either are superparamagnetic or have a low Curie point such that the particles have low permanent magnetization. The coated particles are prepared by emulsifying an aqueous solution or dispersion of magnetic material and an aqueous solution or sol of the inorganic oxide in an inert, water-immiscible liquid. The aqueous droplets that form are gelled, recovered, and heated at 250.degree. C. to 2000.degree. C. The coating is continuous and covers the entire surface of the particle core to prevent exposure of the core to the surrounding media. The coated particles typically have a magnetic material content of below 50% by weight. Example 7 of U.S. Pat. No. 5,039,559 describes an Al.sub.2 O.sub.3 /LiFe.sub.5 O.sub.8 coating having a thickness of 10 nanometers.
U.S. Pat. No. 4,400,432 describes treated, ferromagnetic iron oxide particles in which the particles are coated with one layer of oxygen-containing anions and a further layer of polyvalent cations. The polyvalent cation layer may be applied between two oxygen-containing anion layers. The materials used to form the layers include water-soluble aluminum salts.
U.S. Pat. No. 4,229,234 describes the preparation of passivated particulate high Curie temperature magnetic alloys. An element, such as aluminum, is alloyed with iron, cobalt, or nickel. The alloy is then formed into particles, and the particles are then exposed to controlled amounts of oxygen at elevated temperature over selected time periods. An oxide of the element forms and diffuses to the surface of the particles to form a film. The films typically have a thickness in the range from 0.1 to 0.5 .mu.m.
U.S. Pat. No. 4,512,682 describes the formation of insulating layers comprising alternating layers of alumina and silica coated on a ferrite substrate.
U.S. Pat. No. 4,336,310 describes silica coated metallic magnetic powders and their use in recording media, which have further layers of silanes and oleic acid coated thereon.
U.S. Pat. Nos. 4,133,677, 4,280,918, 4,309,459 and 4,321,303 describe iron oxide fine powders having a silica layer formed on the surface thereof.
U.S. Pat. No. 3,535,245 describes metal oxide coated ferromagnetic particles prepared by a microgel process involving epoxy materials, an alumina gel, and ferromagnetic particles.
U.S. Pat. No. 4,944,802 describes the use of barium ferrite pigment particles in combination with inorganic oxide particles dispersed in a binder. The inorganic oxide particles are used as an abrasive and are not used to coat the barium ferrite pigment particles.
Japanese Kokai No. 1-119,519 describes magnetic particles coated with a layer of hydrated alumina with a boehmite structure. A coupling agent layer is then further deposited onto the surface of the coated particles.
Similarly, Japanese Kokai No. 1-176,229 describes magnetic particles having hydrated alumina shells composed of boehmite particles that are further overcoated with a layer of silica. It also describes magnetic recording media containing such coated particles.
Japanese Kokai No. 61-222,207 describes magnetic particles coated with aluminum oxide alumina sol shells useful in compression molding of magnetic objects. The aluminum oxide shell thicknesses were in the range of 1 to about 10 microns, with the thicker layers being preferred.
WIPO published patent application 90-15,364 describes a process for making non-smearable colored magnetic particles that comprises the steps of providing magnetic core particles of metal or reducible metal oxide 1-50 microns in size, depositing submicron particles of a non-reducible oxide of a different metal on the core particles, heating the aggregate particles in an oxygen-containing atmosphere to oxidize the surface of the core particles, and heating the particles sufficiently in an inert atmosphere to cause reaction between the core surface oxide and the deposited oxide, thus forming the surface colored magnetic core particles.
U.S. Pat. No. 2,085,129 describes the production of colloidal metal hydroxides. The colloidal metal hydroxides can be obtained by causing salts of trivalent metals, e.g., aluminum, to act in approximately stoichiometrical ratio on agents that decompose the salts to form hydroxides. The sols obtained by this process, e.g., alumina sols, are described as being useful as protective colloids (See page 4, first column, lines 33-40).
The adsorption of a polynuclear cation or positively charged colloid onto another positively charged colloid has been described in (a) "A Surface Precipitation Model for the Sorption of Cations on Metal Oxides", Farley, K. J.; Dzombak, D. A.; Morel, F. M. M. J. Colloid Interface Sci. 1985, 106, 226; (b) "Adsorption of Al(III) at the TiO.sub.2 -H.sub.2 O Interface", Wiese,G. R.; Healy, T. W. J. Colloid Interface Sci. 1975, 51, 434; and (c) "Adsorption of Hydrolyzable Metal Ions at the Oxide-Water Interface", James, R. O.; Healy, T. W. J. Colloid Interface Sci. 1972, 40, 65.
Some of these prior art techniques rely on crystalline materials, e.g., boehmite and pseudoboehmite, to coat magnetic particles. Crystalline coating materials, however, tend to provide coatings that are discontinuous and/or too thick. With a discontinuous coating, only part of the surface of the magnetic particles is covered by the coating materials. Unless the particles are further coated by layers of additional coating materials, discontinuous coatings provide particles having two different surfaces which must be wetted in order to disperse the particles. Discontinuous coatings, therefore, provide magnetic particles that are more difficult to disperse than particles having only a single kind of surface for wetting. When the coating is too thick, e.g., having a thickness of greater than 5 nm, the tendency of the magnetic particles to form aggregates may be reduced. However, the packing density of the magnetic particles is also significantly reduced. In view of this reduction in packing density, magnetic particles having such thick coatings would be less suitable for high density magnetic recording applications relative to magnetic particles having thinner coatings.
Other prior art techniques rely on gelation to coat the magnetic particles and are capable of providing continuous coatings. However, such coatings tend to coat large agglomerates of particles rather than individual particles and also tend to provide coatings that are too thick.
Other prior art techniques depend upon electrostatic attraction between the coating materials and the magnetic particles to achieve coating. This approach, however can only be used with certain kinds of magnetic particles having the proper surface charge characteristics. Moreover, this approach often requires a pH change to achieve coating. Changing the pH to achieve coating may destabilize the aqueous magnetic suspension and can cause poor quality, thick coatings to form on the magnetic particles. This approach can also degrade the magnetic properties of the magnetic particles.
Other miscellaneous coating techniques have also been described. What is needed in the art, however, are improved coating techniques that allow thin (i.e., 5 nm or less), continuous coatings to be formed on any kind of magnetic particle, regardless of its surface charge characteristics and without requiring gelation or a pH change to effect such coating.